But the differences between the two sides were laid bare earlier in the evening when they addressed reporters.

Kerry struck a musclular tone, demanding the Syrian government quickly turn over data about its chemical weapons program and rejecting demands from Syrian President Bashar al-Assad that the U.S. back off its threat of force.

"President Obama has made clear that should diplomacy fail, force might be necessary to deter and degrade Assad's capacity to deliver these weapons," Kerry said.

Russia has been opposed to American threats to strike inside Syria. On Thursday, Lavrov said he hoped the negotiations here would make those threats unnecessary.

Syria submitted paperwork to join the Chemical Weapons Convention on Thursday, the first step in giving up its chemical weapons. Assad said he would submit a full accounting of his stockpiles in 30 days. Kerry rejected that, saying it was too slow.

"There is nothing standard about this process at this moment," he said. "Words of the Syrian regime, in our judgment, are simply not enough."

Today, experts from both sides are meeting to hammer out the nuts and bolts of how a transfer plan would be implemented. Intelligence analysts will compare notes on the scope of Assad's toxic gas stockpiles and technical teams will discuss how the work would be carried out in the middle of a war zone.

Earlier this morning, Kerry and Lavrov met with U.N. Special Envoy for Syria Lakhdar Brahimi. Brahimi urged the two sides to recommit to organizing a long-delayed international peace conference.

Kerry said the prospects of such a conference will depend on the success of the chemical weapons plan and Assad's willingness to implement it.

"We are working hard to find common ground to be able to make that happen. And we discussed some of the homework that we both need to do. I'm not going to go into it in any detail today," he said. "Much of which will obviously depend on the capacity to have success here in the next days, hours, days on the subject of the chemical weapons."

The three plan to meet again later this month in New York, when they'll all be in town to attend the annual United Nations General Assembly meeting.